Optical communications via fiber networks can be adversely affected by certain physical properties or processes in optical fiber, such as optical attenuation, chromatic dispersion (CD), polarization mode dispersion (PMD), optical phase noise (PN) and nonlinear optical effects in optical fiber. Coherent optical detection technologies have been developed to mitigate those adverse effects in which a local oscillator (LO) laser (e.g., a continuous-wave laser) is used in an optical receiver to coherently combine with the received optical transmission signal before being detected by an optical detector. This coherent optical detection with the LO laser is used to obtain the optical phase information in the received optical transmission signal.
Coherent optical detection can be used to increase more information to be carried in one optical channel than the corresponding direct detection. Because of that advantage, coherent optical detection can be used to improve long hall communications via fiber links. For optical communications over shorter distances that are much more sensitive to the price, footprint, and power dissipation, coherent optical detection technologies are not presently used in most commercial deployment. Coherent optical detection technologies also allow compensate for such deteriorating effects as Chromatic Dispersion (CD), Polarization Mode Dispersion (PMD), and frequency dependent losses in electrical and opto-electrical elements. However,
One of the approaches to coherent optical detection is using digital signal processing (DSP) in the digital domain to reduce adverse impacts of the transmission system impairments.